An inkjet printing system typically includes one or more printheads and their corresponding ink supplies. A printhead includes an ink inlet that is connected to its ink supply and an array of drop ejectors, each ejector including an ink pressurization chamber, an ejecting actuator and a nozzle through which droplets of ink are ejected. The ejecting actuator may be one of various types, including a heater that vaporizes some of the ink in the chamber in order to propel a droplet out of the nozzle, or a piezoelectric device that changes the wall geometry of the ink pressurization chamber in order to generate a pressure wave that ejects a droplet. The droplets are typically directed toward paper or other print medium (sometimes generically referred to as recording medium or paper herein) in order to produce an image according to image data that is converted into electronic firing pulses for the drop ejectors as the print medium is moved relative to the printhead.
Motion of the print medium relative to the printhead can include keeping the printhead stationary and advancing the print medium past the printhead while the drops are ejected. This architecture is appropriate if the nozzle array on the printhead can address the entire region of interest across the width of the print medium. Such printheads are sometimes called pagewidth printheads. A second type of printer architecture is the carriage printer, where the printhead nozzle array is somewhat smaller than the extent of the region of interest for printing on the print medium and the printhead is mounted on a carriage. In a carriage printer, the print medium is advanced a given distance along a print medium advance direction and then stopped. While the print medium is stopped, the printhead carriage is moved in a carriage scan direction that is substantially perpendicular to the print medium advance direction as the drops are ejected from the nozzles. After the carriage has printed a swath of the image while traversing the print medium, the print medium is advanced, the carriage direction of motion is reversed, and the image is formed swath by swath.
Inkjet ink includes a variety of volatile and nonvolatile components including pigments or dyes, humectants, image durability enhancers, and carriers or solvents. A key consideration in ink formulation and ink delivery is the ability to produce high quality images on the print medium. Image quality can be degraded if air bubbles block the small ink passageways from the ink supply to the array of drop ejectors. Such air bubbles can cause ejected drops to be misdirected from their intended flight paths, or to have a smaller drop volume than intended, or to fail to eject. Air bubbles can arise from a variety of sources. Air that enters the ink supply through a non-airtight enclosure can be dissolved in the ink, and subsequently be exsolved (i.e. come out of solution) from the ink in the printhead at an elevated operating temperature, for example. Air can also be ingested through the printhead nozzles. For a printhead having replaceable ink supplies, such as ink tanks, air can also enter the printhead when an ink tank is changed.
In a conventional inkjet printer, a part of the printhead maintenance station is a cap that is connected to a suction pump, such as a peristaltic or tube pump. The cap surrounds the printhead nozzle face during periods of nonprinting in order to inhibit evaporation of the volatile components of the ink. Periodically, the suction pump is activated to remove ink and unwanted air bubbles from the nozzles. This pumping of ink through the nozzles is not a very efficient process and wastes a significant amount of ink over the life of the printer. Not only is ink wasted, but in addition, a waste pad must be provided in the printer to absorb the ink removed by suction. The waste ink and the waste pad are undesirable expenses. In addition, the waste pad takes up space in the printer, requiring a larger printer volume. Furthermore the waste ink and the waste pad must be subsequently disposed. Also, the suction operation can delay the printing operation
Co-pending US Patent Application Publication No. 2011/0209706 entitled “Air Extraction Device for Inkjet Printhead” discloses an inkjet printhead including an air extraction chamber having a compressible member for forcing air to be vented from an air chamber through a one-way relief valve in its open position, and for applying a reduced air pressure to a membrane while the one-way relief valve is in its closed position. The compressible member, for example a bellows, is compressed by a projection from a wall of the printer when the carriage moves to an end of travel. Co-pending U.S. patent application Ser. No. 13/095,998 filed on Apr. 28, 2011, is a related design that uses a piston assembly rather than a compressible member, the piston being moved to a first position by a projection from a wall of the printer when the carriage moves to an end of travel. Both of these air extraction devices are actuated by moving the carriage to an end of travel. Both of these copending patent applications are incorporated by reference herein in their entireties.
U.S. Pat. No. 6,116,726, entitled “Ink Jet Printer Cartridge with Inertially-Driven Air Evacuation Apparatus and Method”, discloses an inkjet printhead (or pen) including a movable inertia element connected to the body of the printhead. The body defines an ink chamber and an air outlet. A compressor element is connected to the inertia element and the air outlet. When the printhead is accelerated along the carriage path during printing, the resulting motion of the inertia element operates the compressor to pump a small amount of air from the chamber. Such a pump is actuated as the carriage moves back and forth during the normal printing process and does not require the carriage to move to an end of travel in order to encounter a projection from a carriage wall. However, the design of the compressor element is somewhat complex.
What is needed is an air extraction device for an inkjet printhead that is actuated as the carriage moves back and forth during the normal printing process, but has a simpler design.